1) Field of the Invention
The invention relates to thickeners based on carboxyl- and carboxamido-containing addition polymers, to their use in coating compositions, adhesives and hydraulically setting binders, and to processes for their preparation.
2) Background Art
In addition to polyurethane thickeners and celluloses, carboxyl- and carboxamido-containing aqueous copolymer dispersions are widely employed as thickeners. They develop their thickening action only in an alkaline medium following neutralization of the carboxyl groups. In order to achieve a good thickening effect, high molecular weights are required in these polymers. A disadvantage of these products is that they possess very high viscosities in neutralized form but in acidic form are not stable on storage. In order to avoid these disadvantages, the following procedures are known.
In EP-A 529 206 (U.S. Pat. No. 5,545,688) polyacrylamide is prepared by inverse emulsion polymerization. This technique can be transferred to carboxylate-containing polymers, and in DE-A 3520507 (U.S. Pat. No. 4,681,912) a water-in-oil microemulsion comprising an aqueous acrylamide/acrylic acid solution, an organic solvent and a nonionic surfactant is polymerized. A disadvantage of this method, however, is that large amounts of organic solvent must be used in order to ensure the phase inversion and must then be separated off again subsequently, which is complex. These organic solvents, such as Isopar.RTM.M and toluene, moreover, are nowadays unacceptable on environmental grounds.
According to EP-A 736 547 (CA-A 2173297) the problems set out above can be avoided by preparing conventional emulsion polymers from water-soluble and water-insoluble monomers, but again in the presence of organic solvents which must subsequently be separated off by distillation. A further disadvantage is that in order to obtain stable dispersions use is made in addition of surfactant monomers which act as associatively thickening groups.
EP-A 450 437 (U.S. Pat. No. 5,231,145) describes copolymers based on alkyl acrylates and/or alkyl methacrylates, prepared with polyvinyl alcohol as protective colloid, for use as sizes. The products, however, are unsuitable for use as thickeners since they show no increase in viscosity on neutralization.
The object of EP-A 627 450 was to prepare thickeners based on (meth)acrylate-(meth)acrylic acid copolymers which are present in acidic form as a stable dispersion and exhibit no WeiBenberg effect, without using the toxic compound ethyl acrylate. Said object was achieved by means of polyvinyl alcohol-stabilized, (meth)acrylate-based copolymer dispersions which are described as being suitable thickeners for aqueous dispersions, especially emulsion paints. The thickening effect, however, is not sufficient to provide the required stability in hydraulically setting compositions.
Thickeners obtainable commercially include those based on acrylamide-acrylic acid copolymers prepared by the technique of precipitation polymerization. A disadvantage of such a thickener is that, in the acidic state in which the product is obtained, its storage stability is unsatisfactory, and without stirring the product rapidly forms a sediment and an agglomerating product cake which can no longer be stirred up or disrupted. For this reason the product is converted at an early stage into the neutralized form, in which it is indeed storage-stable but has a very high viscosity (10,000 mPas at 7% solids content) and can therefore be prepared only in very highly diluted form, and hence uneconomically.
The object was therefore to provide storage-stable, water-soluble polymers of low viscosity which can be employed as thickeners.